Exhaust gas purification device for an internal combustion engine and exhaust gas purification method for an internal combustion engine

ABSTRACT

An internal combustion engine with an exhaust gas purification device is provided in which the NOx held in a NOx catalyst can be efficiently reduced and purified, and a sufficient amount of NOx can be reduced, thereby making it possible to regenerate the NOx catalyst over a wide range thereof. When processing of releasing and reducing the NOx held in the NOx catalyst  33  for purification, light oil is injected by an addition valve  37  so as to be supplied to the NOx catalyst  33  together with the exhaust gas, and after the droplet-like light oil adheres to the entire area of the NOx catalyst  33 , the flow rate of the exhaust gas is decreased.

TECHNICAL FIELD

The present invention relates to an exhaust gas purification device foran internal combustion engine and an exhaust emission control method foran internal combustion engine for purifying NOx components contained inan exhaust gas.

BACKGROUND ART

In the past, there has been known an exhaust gas purification device foran internal combustion engine that is provided with a NOx storagereduction catalyst to occlude and reduce NOx for purifying NOxcomponents in an exhaust gas (see, for example, a first patent document(Japanese patent application laid-open No. 2000-240428), a second patentdocument (Japanese patent application laid-open No. H6-200740), a thirdpatent document (Japanese patent application laid-open No. 2000-345831),and a fourth patent document (Japanese patent application laid-open No.S62-106826)). In such an exhaust gas purification device, a reducingagent is supplied to the NOx catalyst at appropriate times, so that NOxcomponents contained in or held by the NOx catalyst are thereby reducedto be purified, thus regenerating the NOx catalyst.

Here, as methods for supplying the reducing agent to the NOx catalyst,in general, there are the following cases: that is, one case is that aliquid reducing agent is evaporated and then supplied in its gaseousstate; and another case is that a liquid reducing agent is supplied inits liquid or droplet state. In the case of supplying a reducing agentin its gaseous state, there is a merit that a desired area can be putinto a reducing atmosphere in a short period of time, but there is ademerit that it is impossible to reduce and purify the NOx held in theNOx catalyst unless the entire NOx catalyst has to be put into areducing atmosphere. In contrast to this, in the case of supplying areducing agent in its droplet state, there is a merit that it ispossible to reduce the NOx held in the NOx catalyst by locally creatinga reducing atmosphere without the need to put the entire NOx catalystinto a reducing atmosphere.

However, in the case of supplying a droplet-like reducing agent, therearises a problem that it is difficult to locally create a reducingatmosphere, so it becomes difficult to reduce and purify the NOx held inthe NOx catalyst to a satisfactory extent. Here, note that if the amountof reducing agent supplied is too large, it is released or emitted tothe atmosphere as it is without being adhered to the NOx catalyst, sothe amount of the reducing agent to be supplied must be limited.

DISCLOSURE OF THE INVENTION

Accordingly, one object of the present invention is to reduce and purifythe NOx held in a NOx catalyst in an efficient manner.

Another object of the present invention is to reduce and purify asufficient amount of the NOx held in the NOx catalyst.

A further object of the present invention is to regenerate the NOxcatalyst over a wide range thereof.

In order to solve the above-mentioned problems or objects, the presentinvention adopts the following solution.

That is, in the present invention, there is adopted a construction thatafter a liquid or droplet-like reducing agent has spread (adhered) tothe entire NOx catalyst, the flow rate of an exhaust gas flowing throughthe NOx catalyst is decreased (including the case where the flow rate isreduced to zero).

According to such a construction of the present invention, the flow rateof the exhaust gas is not decreased at the time when the reducing agentis being supplied, so it is possible to easily supply the reducing agentto the whole from an upstream side to a downstream side of the NOxcatalyst in a uniform manner. That is, the reducing agent is carriedalong with the exhaust gas, so in a state where the flow rate of theexhaust gas is decreased, it becomes difficult to supply the reducingagent to the downstream side of the NOx catalyst. In contrast to this,in the present invention, the reducing agent is supplied with the flowrate of the exhaust gas being not decreased, and hence the reducingagent can be supplied to the downstream side to a satisfactory extent.In addition, since the flow rate of the exhaust gas is decreased afterthe reducing agent has spread to the entire NOx catalyst, it is possibleto widen an area of a reducing atmosphere formed around the droplet-likereducing agent adhered to the NOx catalyst as well as to keep thereducing atmosphere for a long period of time. That is, the reducingagent adhered to the NOx catalyst is evaporating, a reducing atmosphereis formed around the NOx catalyst during the progress of evaporation.Here, the gas, which forms the reducing atmosphere around thedroplet-like reducing agent, is caused to flow along with the exhaustgas (which is not the reducing atmosphere). Accordingly, the less theflow rate of the exhaust gas, the wider the range of the reducingatmosphere can be made, and the longer in time the reducing atmospherecan be kept. Moreover, the flow rate of the exhaust gas is small, theregion of the reducing atmosphere is wide, and the reducing atmospherecontinues for a long period of time, as a result of which thetemperature of the NOx catalyst rises quickly or at an early time. Thus,the NOx releasing and reducing speed or rate due to the NOx catalyst areincreased, and the efficiency of purifying the NOx is raised in asynergistic manner.

As a more specific exhaust gas purification device for an internalcombustion engine, according to the present invention, there is providedan exhaust gas purification device for an internal combustion engine inwhich a reducing agent supply means is disposed on an exhaust passagefor supplying a droplet-like reducing agent to a NOx storage reductioncatalyst, which serves to occlude and reduce NOx components in anexhaust gas, from its upstream side, so that the NOx components held insaid NOx catalyst are reduced and purified by the reducing agentsupplied thereto from said reducing agent supply means, said devicebeing characterized by comprising:

a determination means for determining whether the droplet-like reducingagent supplied by said reducing agent supply means has spread to atleast a predetermined range; and

an adjustment means for adjusting a flow rate of the exhaust gas sent tosaid NOx catalyst;

wherein the flow rate of the exhaust gas is decreased by said adjustmentmeans when said determination means makes a determination that thereducing agent has spread.

Here, it is preferable that the predetermined range is an entire rangeof the NOx catalyst, but it is not necessarily so. Further, in thepresent invention, even after the processing of decreasing the flow rateof the exhaust gas according to the adjustment means is started, thesupply of the reducing agent may be continued. Moreover, as theadjustment means for the flow rate of the exhaust gas, there areenumerated, for example, a construction in which a plurality of exhaustgas passages are provided in such a manner that the amount of exhaustgas supplied to each passage is changed by a valve or the like, aconstruction that adopts a variable valve system, a construction inwhich the amount of intake air and/or the amount of exhaust gas areadjusted by intake and/or exhaust valves, a construction in which theamount of EGR is adjusted by an EGR valve, and a construction in whichthe amount of intake air is adjusted by a throttle valve. In addition,fuel (light oil in case of a diesel engine) is enumerated as a suitableexample of the reducing agent.

According to such a construction of the present invention, the flow rateof the exhaust gas is not decreased at the time when the reducing agentis being supplied, so the reducing agent can be easily carried up to thedownstream side of the NOx catalyst along with the exhaust gas. As aresult, the reducing agent can be easily supplied to the whole from theupstream side of the NOx catalyst to the downstream side thereof.Accordingly, the reducing agent can be easily spread in thepredetermined range in a uniform manner. In addition, since the flowrate of the exhaust gas is decreased after the reducing agent has spreadin the predetermined range, it is possible to widen an area of thereducing atmosphere formed around the droplet-like reducing agentadhered to the NOx catalyst as well as to keep the reducing atmospherefor a long period of time. Further, the temperature of the NOx catalystgoes up quickly or at an early stage, so that the releasing and reducingspeed or rate of the NOx due to the NOx catalyst are increased.

Moreover, when said determination means makes a determination that thereducing agent has spread, the supply of the reducing agent by saidreducing agent supply means may be stopped, and thereafter the flow rateof the exhaust gas may be decreased by said adjustment means.

By doing so, the reducing agent can be prevented from being consumedmore than necessary. In particular, even in case where a reducing agentcontaining HC (e.g., fuel) is used, it is possible to suppress the HCfrom being emitted or released to the atmosphere.

An element which becomes a determination reference or criterionaccording to said determination means may include at least one of theNOx purification ratio of said NOx catalyst, the amount of the HCemitted or exhausted to the downstream side of said NOx catalyst, thetemperature of said NOx catalyst, the time elapsed from the start ofsupply of the reducing agent by said reducing agent supply means, andthe flow rate of the exhaust gas that has passed through a unit volumeof the catalyst within a unit time.

Here, when the NOx purification rate is used as an element that becomesa determination reference or criterion, it is possible to recognize,from the NOx purification rate after the processing of reducing andpurifying the NOx held in the NOx catalyst is carried out by supplyingthe reducing agent, whether the reducing agent has spread in thepredetermined range. Accordingly, the reducing agent can be made tospread in the predetermined range in an appropriate manner by performingso-called feedback control in which the supply time of the reducingagent is corrected when the following supply of the reducing agent iscarried out. Here, note that the NOx purification rate means the ratioof a portion of the NOx purified by the NOx catalyst to the entire NOxexhausted from cylinders. For example, this NOx purification rate can becalculated, for example, from the results of detection of a pair of NOxsensors arranged at the upstream and downstream sides, respectively, ofthe NOx catalyst.

In addition, in the case of using, as an element for a determinationcriterion, the HC exhausted to the downstream side of the NOx catalyst,when it is detected that HC has been exhausted to the downstream side ofthe NOx catalyst, or when the amount of the HC exhausted to thedownstream side of the NOx catalyst exceeds a predetermined amount, itcan be determined that the reducing agent has spread in thepredetermined range. The detection of HC can be performed by using an HCsensor. Here, note that in the case of using the HC as an element for adetermination criterion, it is required that HC is contained as acomponent for the reducing agent.

Moreover, in the case of using the temperature of the NOx catalyst as anelement for a determination criterion, when the temperature of the NOxcatalyst exceeds a predetermined temperature (a preset temperature, or atemperature determined based on the reference temperature inconsideration of other conditions), it is possible to determine that thereducing agent has spread in the predetermined range. In thisconnection, it is to be noted that the temperature of the NOx catalystcan be detected directly by the use of a temperature sensor, orestimated from a temperature at another location.

Moreover, in the case of using, as an element for a determinationcriterion, the time elapsed from the start of supply of the reducingagent by the reducing agent supply means, when the elapsed time exceedsa predetermined time, it can be determined that the reducing agent hasspread in the predetermined range. In this regard, note that the elapsedtime can be measured with the use of a timer. Here, a preset referencetime, a time determined based on a reference time in consideration ofother conditions or the like can be used as said predetermined time, andthe flow rate of the exhaust gas (SV) having passed through the unitvolume of the catalyst per unit time is referred to as a suitableexample for the other conditions.

Here, note that the determination may be made by using only one of theseelements for determination criteria, or by properly using two or moreelements in a comprehensive manner.

In addition, it is preferable that a second determination means isprovided for determining whether the adjustment of decreasing the flowrate of the exhaust gas by said adjustment means is to be terminated.

According to the above construction of the present invention, theprocessing of decreasing the flow rate of the exhaust gas can beterminated when appropriate. Accordingly, it is possible to return theflow rate of the exhaust gas to an ordinary level at the earliestpossible stage.

The element which becomes a criterion for the determination of saidsecond determination means may include at least one of the NOxpurification rate of said NOx catalyst, the HC exhausted to thedownstream side of said NOx catalyst, the temperature of said NOxcatalyst, the time elapsed from the start of the adjustment ofdecreasing the flow rate of the exhaust gas by said adjustment means,and the flow rate of the exhaust gas having passed through the unitvolume of said catalyst per unit time.

Here, in the case of using the NOx purification rate as an element for adetermination reference or criterion, it is possible to recognize expost facto, from the NOx purification rate after the processing ofreducing and purifying the NOx held in the NOx catalyst is carried outby supplying the reducing agent, whether a time duration for which theflow rate of the exhaust gas has been decreased is appropriate.Accordingly, it is possible to correct the time duration in anappropriate manner by performing so-called feedback control in which thetime duration is corrected when the reducing agent is supplied at thenext time.

In addition, in the case of using, as an element for a determinationcriterion, the HC exhausted to the downstream side of the NOx catalyst,when HC is stopped being exhausted to the downstream side of the NOxcatalyst or when the amount of the HC exhausted to the downstream sideof the NOx catalyst becomes less than a predetermined amount, it can bedetermined that the processing of reducing the flow rate of the exhaustgas is to be terminated.

Moreover, in the case of using the temperature of the NOx catalyst as anelement for a determination reference, when the temperature of the NOxcatalyst becomes less than a predetermined temperature (a presetreference temperature, a temperature determined based on a referencetemperature in consideration of other conditions, etc.), it can bedetermined that the processing of reducing the flow rate of the exhaustgas is to be terminated.

Further, in the case of using, as an element for a determinationcriterion, the time elapsed from the start of the adjustment ofdecreasing the flow rate of the exhaust gas by said adjustment means,when the elapsed time exceeds a predetermined time (a secondpredetermined time), it can be determined that the processing ofdecreasing the flow rate of the exhaust gas is to be terminated. Here, apreset reference time, a time determined based on a reference time inconsideration of other conditions or the like can be used as saidpredetermined time (the second predetermined time), and a flow rate ofthe exhaust gas (SV) having passed through the unit volume of thecatalyst per unit time is enumerated as a suitable example for the otherconditions.

Here, note that the determination may be made by using only one of theseelements for determination criteria, or by properly using two or moreelements in a comprehensive manner.

Also, it is preferable that the lower the temperature of said NOxcatalyst, the more the flow rate of the exhaust gas is decreased by saidadjustment means.

As a result, it is possible to properly adjust the flow rate of theexhaust gas in accordance with the temperature of the NOx catalyst. Thatis, the lower the temperature of the NOx catalyst, the more the speed atwhich the NOx held in the NOx catalyst is reduced is decreased.Therefore, the lower the temperature of the NOx catalyst, the higher thenecessity of keeping the reducing atmosphere for a long period of timebecomes. Accordingly, by decreasing the flow rate of the exhaust gas inaccordance with the lowering temperature of the NOx catalyst, it becomespossible to keep the reducing atmosphere for the longer period of time,whereby the flow rate of the exhaust gas can be adjusted to a levelcorresponding to the temperature of the NOx catalyst.

Further, provision may be made for a first exhaust path and a secondexhaust path arranged at the downstream side of said reducing agentsupply means, with a NOx catalyst being provided on each of said firstand second exhaust paths, and a valve for adjusting the flow rate of theexhaust gas with respect to each of these exhaust paths, wherein whenthe processing of reducing and purifying the NOx held in the NOxcatalysts is not performed, the exhaust gas is caused to flow into bothof the exhaust paths, whereas when said purification processing isperformed, the supply of the reducing agent to one of said NOx catalystsby means of said reducing agent supply means is started with the exhaustgas being controlled by said valve to flow only into that one of saidexhaust paths in which the one of said NOx catalysts to be processed forpurification is arranged, and when the processing of decreasing the flowrate of the exhaust gas by means of said adjustment means is performed,the exhaust gas is controlled to flow into the other of said exhaustpaths by said valve, whereby the flow rate of the exhaust gas to the oneof said exhaust paths in which the one of said NOx catalysts to beprocessed for purification is arranged is decreased.

According to such a construction of the present invention, the exhaustpassage is constituted by a plurality of paths in such a manner that theflow rate of the exhaust gas to each path can be properly changed,thereby achieving the decreasing processing of the flow rate of theexhaust gas. Additionally, when the processing of reducing and purifyingthe NOx is not performed, the exhaust gas is caused to flow into both ofthe first exhaust path and the second exhaust path in which the NOxcatalysts are arranged, respectively. Accordingly, the NOx catalystsarranged in the exhaust paths, respectively, are both used, so there isno particular need to increase the capacity of the catalysts. Moreover,when the processing of reducing and purifying the NOx is performed, thereducing agent is supplied only to the one of the exhaust paths in whichthe one of said NOx catalysts to be processed is arranged. Accordingly,the reducing agent can be used without waste. Further, when theprocessing of decreasing the flow rate of the exhaust gas is performed,the flow rate of the exhaust gas to the other of said exhaust paths isincreased by the valve, whereby the flow rate of the exhaust gas to theone of said exhaust paths in which the one of said NOx catalysts to beprocessed for purification is arranged is decreased. Accordingly, theprocessing of decreasing the flow rate of the exhaust gas to the one ofsaid NOx catalysts to be purified can be done without changing the totalamount of the flow rate of the exhaust gas.

Furthermore, when SOx held in the NOx catalysts is reduced and purified,and when particles adhered to the NOx catalysts, which also have afilter function, are oxidatively removed, processing of increasing anddecreasing the flow rate of the exhaust gas flowing through that one ofsaid exhaust paths in which the one of said NOx catalysts to beprocessed for purification is arranged is performed by said valve atleast one time.

By doing so, it is possible to perform the reduction purification of theSOx or the oxidation removal of the particles over the entire area ofthe NOx catalysts in a suitable manner. That is, when these processingoperations are performed, it is necessary to raise the temperature ofthe NOx catalysts to a value equal to or more than a predeterminedtemperature. In order to carry out these processing operations over theentire area of the NOx catalysts, the temperature of the entire area ofthe NOx catalysts must be raised to a value equal to or more than thepredetermined temperature. Here, when the flow rate of the exhaust gasis small, the reducing agent is mainly supplied to the upstream side ofthe NOx catalysts, and hence the temperature of the NOx catalysts at theupstream side thereof becomes high mainly by the reductive reaction ofsaid reducing agent, whereas when the flow rate of the exhaust gas islarge, a lot of reducing agent is supplied to the downstream side of theNOx catalysts, so the temperature of the NOx catalysts at the downstreamside thereof also becomes high by the reductive reaction of the reducingagent. Therefore, by performing the increasing and decreasing processingof the flow rate of the exhaust gas at least one time, the temperatureof the NOx catalysts can be raised all around from the upstream side tothe downstream side thereof.

In addition, said valve comprises a switch valve that is able to switchthe path through which the exhaust gas flows to the first exhaust pathor the second exhaust path,

said increasing and decreasing processing is performed by said switchvalve that alternately switches the path through which the exhaust gasflows between said first and second exhaust paths, and

the timing at which the reducing agent is supplied by said reducingagent supply means is synchronized with the timing at which the paththrough which the exhaust gas flows may be switched by said switchvalve.

By doing so, it is possible to appropriately carry out the increasingand decreasing processing of the flow rate of the exhaust gas withrespect to both of the first exhaust path and the second exhaust path.

Moreover, an exhaust gas purification method for an internal combustionengine for purifying NOx contained in an exhaust gas according to thepresent invention comprises:

a step of making a droplet-like reducing agent adhere to a NOx storagereduction catalyst by supplying a reducing agent from an upstream sideof said NOx catalyst that occludes and reduces NOx; and

a step of decreasing the flow rate of the exhaust gas sent to the NOxcatalyst after it is determined by a determination means that thedroplet-like reducing agent has spread in at least a predetermined rangein the NOx catalyst.

Here, note that the above-mentioned respective constructions can beadopted in combination with one another wherever possible.

As described in the foregoing, according to the present invention, it ispossible to reduce and purify the NOx held in the NOx catalysts in anefficient manner. Also, it is possible to reduce and purify a sufficientamount of the NOx held in the NOx catalysts. In addition, the NOxcatalysts can be regenerated over a wide range thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an overall schematic construction view of an internalcombustion engine provided with an exhaust gas purification device.

FIG. 2A is a view explaining a droplet-like reducing agent (in case of alarge amount of SV).

FIG. 2B is a view explaining the droplet-like reducing agent (in case ofa small amount of SV).

FIG. 3 is a graphic representation illustrating the relation between thetemperature of a NOx catalyst and the speed at which the NOx held in theNOx catalyst is released and reduced.

FIG. 4A is a timing chart (a preferred example) illustrating therelation between a pulse for driving a valve that switches an exhaustpath and a pulse for adding the reducing agent.

FIG. 4B is a timing chart (an inappropriate example) illustrating therelation between a pulse for driving the valve that switches the exhaustgas path and a pulse for adding the reducing agent.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, the best mode for carrying out the present invention will bedescribed below in detail, by way of example, based on the followingembodiment while referring to the accompanying drawings. However, it isto be understood that the measurements, materials, configurations,relative arrangements and the like of component parts described in thefollowing embodiment are only illustrative but should not be construedas limiting the scope of the present invention in any manner, inparticular unless specified otherwise.

EXAMPLE 1

An exhaust gas purification device for an internal combustion engine andan exhaust emission control method for an internal combustion engineaccording to a preferred embodiment of the present invention will bedescribed with reference to FIG. 1 through FIG. 4. FIG. 1 is a schematicconstruction view of the entire internal combustion engine that isprovided with the exhaust gas purification device. FIG. 2 is anexplanatory view of a droplet-like reducing agent. That is, in FIG. 2,there are illustrated the manner in which the droplet-like reducingagent creates a reducing atmosphere, as well as a portion of the NOxcatalyst to which the droplet-like reducing agent is adhered, and theamount of occlusion of the NOx therearound. Here, note that FIG. 2Ashows the case where SV (the flow rate of the exhaust gas that haspassed through the unit volume of the catalyst per unit time) is large,and FIG. 2B shows the case where SV is small. FIG. 3 is a graphicrepresentation illustrating the relation between the temperature of theNOx catalyst and the speed at which the NOx held in the NOx catalyst isreleased and reduced. FIG. 4 is a timing chart illustrating the relationbetween a pulse for driving a valve that switches between exhaust pathsand a pulse for adding the reducing agent. In this regard, FIG. 4A showsan appropriate or preferred example, and FIG. 4B shows an inappropriateexample.

<Outline Explanation of the Internal Combustion Engine Provided with theExhaust Gas Purification Device>

Now, the outline of the internal combustion engine 100 according to thisembodiment will be described below with reference to FIG. 1. In thisembodiment, description will be made while taking an example of a dieselengine as the internal combustion engine 100. The internal combustionengine 100 according to this embodiment includes an engine proper 10, anintake pipe 20 for supplying fresh air to the engine proper 10, anexhaust gas purification device 30 for purifying an exhaust gasexhausted from the engine proper 10 to release it to the atmosphere, andan exhaust gas recirculation device (EGR device) 40 for returning a partof the exhaust gas to an intake air so as to control the generation ofNOx. The exhaust gas recirculation device 40 is provided with an EGRcooler 41 for cooling the returned exhaust gas (EGR gas).

<Explanation of the Exhaust Gas Purification Device>

The exhaust gas purification device 30 is provided with two exhaustpaths, i.e., a first exhaust path 31 and a second exhaust path 32, in anexhaust pipe. NOx storage reduction catalysts 33, 34 are arranged inthese exhaust paths, respectively. As an concrete example for these NOxcatalysts, there are enumerated a NOx storage reduction catalyst, and aparticulate filter carrying such a NOx storage reduction catalyst. Inaddition, a switch valve 35 capable of controlling the flow rate of theexhaust gas to these exhaust gas paths is arranged at a branch portionupstream of these exhaust gas paths. This switch valve 35 can beswitched between the state in which both the entrance of a channel forthe first exhaust path 31 and the entrance of a channel for the secondexhaust path 32 are opened, and the state in which the entrance of oneflow passage of these exhaust paths is opened, and the entrance of theother flow passage is closed. Also, the switch valve 35 can control theflow rate of the exhaust gas to each of the exhaust paths by adjustingthe open area of the entrance of each flow passage to these exhaust gaspaths.

A temperature sensor 36 is installed on the exhaust gas purificationdevice 30 for measuring the temperatures of the NOx catalysts 33, 34. Inaddition, the addition valve 37 is arranged in an exhaust manifoldupstream of the branch portion of the first exhaust path 31 and thesecond exhaust path 32 for supplying the reducing agent to these exhaustpaths. In this embodiment, the reducing agent supplied by the additionvalve 37 is a fuel (light oil).

<Outline of the Processing for Releasing and Reducing the NOx Held inthe NOx Catalyst>

The NOx storage reduction catalysts 33, 34 according to this embodimenthave a property that they absorb NOx under the condition that theexhaust gas contains a large proportion of oxidative components(oxidative atmosphere), but release NOx to the exhaust gas for reductionunder the condition that the exhaust gas contains a small proportion ofoxidative components with the presence of a reducing agent (HC, etc.)(reducing atmosphere).

Here, note that the NOx catalysts 33, 34 come to absorb NOx no more whena predetermined limit of NOx is absorbed. Accordingly, control forrecovering the NOx absorption capabilities of the NOx catalysts 33, 34is repeated at predetermined intervals by purifying the NOx catalysts33, 34 through the release and reduction of the NOx held therein. Suchcontrol is performed based on a NOx purification rate, an operatinghistory, etc.

When the processing of releasing and reducing the NOx held in the NOxcatalysts 33, 34 is carried out, light oil, which serves as a reducingagent, is injected by the addition valve 37. The droplet-like light oilthus injected is carried to the downstream side of the exhaust pathstogether with the exhaust gas. As a result, the droplet-like light oiladheres to the NOx catalysts 33, 34. The droplet-like light oil adheredto the NOx catalysts 33, 34 is vaporized gradually to form a reducingatmosphere in the surroundings, and the NOx held in the NOx catalysts33, 34 is released and reduced to be purified in a region where thereducing atmosphere has been formed. Here, the longer the time orduration of the reducing atmosphere, the amount of the NOx to bereleased and reduced increases.

<Procedure for Releasing and Reducing the NOx Held in the NOx Catalysts>

In this embodiment, both the entrance of the flow passage for the firstexhaust path 31 and the entrance of the flow passage for the secondexhaust path 32 are opened by the switch valve 35 at the time of normaloperation (when the processing of releasing and reducing the NOx held inthe NOx catalysts is not carried out).

Hereinafter, a procedure for the processing of releasing and reducingthe NOx held in the NOx catalysts will be described in the order ofprocesses to be done. Here, note that the processing of either of theNOx catalyst 33 arranged in the first exhaust path 31 and the NOxcatalyst 34 arranged in the second exhaust path 32 is carried outaccording to the same procedure. Accordingly, reference herein will bemade to only the case where processing of the NOx catalyst 33 arrangedin the first exhaust path 31 is carried out.

<<Procedure>>

First of all, by means of the switch valve 35, the entrance of the flowpassage for the second exhaust path 32 is closed and the entrance of theflow passage for the first exhaust path 31 is opened so that light oilis supplied by being injected from the addition valve 37. The light oilthus injected is carried to the downstream side of the first exhaustpath 31 together with the exhaust gas. As a result, the droplet-likelight oil is adhered to the NOx catalyst 33 arranged in the firstexhaust path 31. Here, in this embodiment, the droplet-like light oil iscarried with the flow rate of the exhaust gas being sufficiently large,so light oil is supplied to the downstream side of the NOx catalyst 33to a satisfactory extent.

When a determination is made by an unillustrated determination sectionthat the light oil has spread in the predetermined range (in thisembodiment, the entire region of the NOx catalyst 33), the supply of thelight oil by the addition valve 37 is stopped. Thereafter, the entranceof the flow passage for the second exhaust path 32 is opened by theswitch valve 35, so that the exhaust gas comes to flow into the secondexhaust path 32, thereby decreasing the flow rate of the exhaust gasflowing into the first exhaust path 31.

Then, when by an unillustrated second determination section thatdetermines whether the adjustment of decreasing the flow rate of theexhaust gas is to be terminated, it is determined that the flow ratedecreasing adjustment is to be terminated, the switch valve 35 returnsto its original position. However, with respect to the NOx catalyst 33arranged in the first exhaust path 31 and the NOx catalyst 34 arrangedin the second exhaust path 32, in general, it is necessary to performthe processing of releasing and reducing the NOx held in the NOxcatalysts at the same time. Accordingly, it is desirable to apply theabove processing to the NOx catalyst 34 continuously after applicationof the processing to the NOx catalyst 33.

<<The Determination Section to Determine Whether the Light Oil hasSpread in the Predetermined Range>>

The determination section to determine whether the light oil has spreadin the predetermined range is one of the functions that an unillustratedcontrol unit (ECU) has for controlling the operation of variouscomponent parts provided of the internal combustion engine 100. The ECUis a device that arithmetically processes electric signals input fromvarious kinds of sensors by means of a microcomputer, and outputselectric signals to various kinds of actuators through an outputprocessing circuit. Here, it is needless to say that the actuators towhich electric signals are output from the ECU after the determinationof the determination section are the addition valve 37 and the switchvalve 35 in this embodiment. Though a variety of techniques can beadopted as such a determination technique of the determination section,some examples thereof will be described herein.

(1) Determination Using the NOx Purification Rate

From the NOx purification rate after the processing of purifying the NOxheld in the NOx catalysts for reduction is carried out, it is possibleto recognize ex post facto whether the light oil acting as the reducingagent had spread in the predetermined range. This is because in general,if the light oil has actually spread in the predetermined range, the NOxpurification rate exceeds a target value, whereas if not, the NOxpurification rate becomes less than the target value. Accordingly, it ispossible to make the light oil spread in the predetermined range in anappropriate manner by performing so-called feedback control in which thesupply time or duration of the light oil is corrected when the light oilis supplied at the next time. Here, note that the NOx purification ratemeans the ratio of that portion of the NOx exhausted from the cylinderswhich is purified (absorbed) by the NOx catalysts among the entire NOx.For example, this NOx purification rate can be calculated from theresults of detection of a pair of NOx sensors arranged at locationsupstream and downstream of the NOx catalysts.

That is, in this case, electric signals are input to the ECU from theNOx sensors arranged upstream and downstream of the NOx catalysts,respectively. The ECU calculates the NOx purification rate from theseinput signals, and when the NOx purification rate thus calculated isless than a target NOx purification rate, it further calculates adifference between of these purification rates, from which the ECU cancalculate a correction value for the light oil supply time when thelight oil is supplied at the next time.

(2) Determination Using HC Exhausted to a Downstream Side of the NOxCatalysts

When it is detected that HC has been exhausted to the downstream side ofeach NOx catalyst or when the amount of the HC exhausted to thedownstream side of each NOx catalyst has exceeded a predeterminedamount, it is possible to determine that the light oil has spread in thepredetermined range. This is because if HC is exhausted to thedownstream side of each NOx catalyst, it is considered that the lightoil has reached a downstream end of each NOx catalyst, and if the amountof the HC exhausted to the downstream side of each NOx catalyst exceedsthe predetermined amount, it is considered that the light oil in eachNOx catalyst has spread to a predetermined extent. Here, note that thedetection of HC can be performed by using an HC sensor.

(3) Determination Using the Temperatures of the NOx Catalysts

When the temperature of each NOx catalyst exceeds a predeterminedtemperature (a preset reference temperature, a temperature determinedbased on a reference temperature in consideration of other conditions,etc.), it is possible to determine that the light oil has spread in thepredetermined range. This is because the temperature of each NOxcatalyst rises in accordance with an increasing area where the light oilhas been supplied. Here, note that the temperature of each NOx catalystcan be detected by the temperature sensor 36.

(4) Determination Using the Time Elapsed

When the time elapsed from the start of supply of the light oil by theaddition valve 37 exceeds a predetermined time, it is possible todetermine that the light oil has spread in the predetermined range. Thisis because the relation between the supply time of the light oil and therange where the light oil has spread can be estimated by experiments andanalyses. In this regard, note that the elapsed time can be measuredwith the use of a timer. Here, a preset reference time, a timedetermined based on a reference time in consideration of otherconditions or the like can be used as the “predetermined time”, and theflow rate of the exhaust gas (SV) having passed through the unit volumeof each catalyst per unit time is enumerated as a suitable example forthe other conditions.

(5) Others

The determination methods (1)-(4) described above can be used singly orindependently, but it is possible to use two or more of thesedetermination methods in combination. For example, by adopting thesedetermination methods (2)-(4), when it is determined according to allthese determination methods that “the light oil has spread in thepredetermined range”, a final determination can be made that “the lightoil has spread in the predetermined range”. In addition, thedetermination method (1) can be combined with either of thedetermination methods (2)-(4). Specifically, in the case of adoptingeither of these determination methods (2)-(4), an error can occur in thedetermination result, so to cope with this, it is possible to make amore appropriate determination by applying the feedback control in (1).

<<Relation Between the Flow Rate of the Exhaust Gas and the Amount ofthe NOx Released from the NOx Catalysts for Reduction>>

The relation between the flow rate of the exhaust gas and the amount ofNOx to be released and reduced from the NOx catalyst will be describedwith particular reference to FIG. 2A and FIG. 2B. In these figures, theappearance of the droplet-like light oil adhered to a surface of a NOxcatalyst is schematically illustrated in an upper portion, and theamount of occlusion of NOx in the NOx catalyst is illustrated in a lowerportion. FIG. 2A indicates the case where SV is large, and FIG. 2Bindicates the case where SV is small.

In these figures, a reference character S designates the surface of theNOx catalyst; a reference character A indicates the droplet-like lightoil adhered to the surface S of the NOx catalyst, and a referencecharacter B indicates a reducing atmosphere region. The droplet-likelight oil A adhered to surface S of the NOx catalyst evaporates from itssurface through vaporization to form the reducing atmosphere range Btherearound. The time or duration for which the state of the reducingatmosphere formed in this manner is maintained is the longest in thecenter (T in these figures) of the droplet-like light oil A adhered tothe surface S of the NOx catalyst, and it shortens or decreases as thedistance from the light oil A increases. Here, note that the partindicated at 0 in these figures is a part in which the formation of thereducing atmosphere is 0 in time. That is, the solid line positionindicated by 0 is a limit position at which the reducing atmosphere canbe formed by the light oil A. Here, note that the amount of the NOx tobe released and reduced from the NOx catalyst increases as the time orduration of the reducing atmosphere increases. Accordingly, a largeamount of NOx is released and reduced in the vicinity of the center ofthe light oil A adhered to the surface of the NOx catalyst (a regionindicated at X in the figures), but the larger the distance from there(a region indicated at Y in the figures), the more insufficient does theamount of the NOx to be released and reduced become, so NOx is notreleased at all in a region (a region indicated at Z in the figures)where no reducing atmosphere is formed.

In this connection, note that a gas forming the reducing atmosphere iscaused to flow along with the exhaust gas, which is an oxidativeatmosphere in the case of the diesel engine. As a result, the larger theflow rate of the exhaust gas, the faster the gas forming the reducingatmosphere is caused to flow. Accordingly, the smaller the flow rate ofthe exhaust gas, the wider the range of the reducing atmosphere can bemade, so the longer in time the reducing atmosphere can be kept. Fromthe above, as can be seen from a comparison between FIG. 2A and FIG. 2B,the smaller the SV, the larger does the amount of NOx to be released andreduced from the NOx catalyst become, and the NOx catalyst can beregenerated over the wider range. In addition, the smaller the SV, thefaster does the temperature of the NOx catalyst rise, so the faster doesthe speed at which the NOx held in the NOx catalyst is released andreduced become, thereby improving the efficiency of releasing andreducing the NOx in a synergistic manner.

<<Adjustment for the Flow Rate of the Exhaust Gas According to theTemperature of the NOx Catalyst>>

As stated above, the NOx catalyst has a property that the speed at whichthe NOx held in the NOx catalyst is released and reduced becomes fasterin accordance with the rising or increasing temperature thereof (seeFIG. 3). Accordingly, in the case of performing the processing ofreleasing and reducing the NOx, the higher the temperature of the NOxcatalyst, the shorter the time for which the reducing atmosphere ismaintained may be, whereas the lower the temperature of the NOxcatalyst, the longer the time for which the reducing atmosphere ismaintained need be. In addition, in case where the temperature of theNOx catalyst is low, it is possible to raise the temperature of the NOxcatalyst at an early stage by increasing the time for which the reducingatmosphere is maintained, as well as making the region of the reducingatmosphere wider.

From the above, in this embodiment, when the flow rate of the exhaustgas is adjusted to decrease, the amount of the decreasing adjustment ischanged in accordance with the temperature detected by the temperaturesensor 36. That is, the lower the detected temperature, the more theflow rate of the exhaust gas is decreased. By doing so, the lower thetemperature of the NOx catalyst, it is possible to increase the time forwhich the reducing atmosphere is maintained, and to make the range ofthe reducing atmosphere wider. As described above, in this embodiment,the flow rate of the exhaust gas can be adjusted to an optimal level inaccordance with the temperature of the NOx catalyst.

<<The Second Determination Section to Determine Whether the Adjustmentof Decreasing the Flow Rate of the Exhaust Gas is to be Terminated>>

When the light oil adhered to the NOx catalyst has fully vaporized(evaporated) and the releasing and reducing processing of the NOx heldin the NOx catalysts is terminated, it is necessary to return the flowrate of the exhaust gas to an original level. Accordingly, by using thesecond determination section which determines whether the adjustment ofdecreasing the flow rate of the exhaust gas is to be terminated, theflow rate of the exhaust gas is returned to the original level when itis determined by the second determination section that the decreasingadjustment is to be terminated. Thus, deterioration of drivability dueto the control of decreasing the flow rate can be suppressed to aminimum by returning the flow rate of the exhaust gas to an ordinarylevel at appropriate timing. The second determination section is one ofthe functions that the ECU has, similar to the above-mentioneddetermination section which determines whether the light oil has spreadin the predetermined range.

Here, note that the NOx purification rate, the HC exhausted to thedownstream side of the NOx catalysts, the temperatures of the NOxcatalysts, the elapsed time, etc., can be used as a determinationtechnique or method according to the second determination section, as inthe case of the determination section for determining whether the lightoil has spread in the predetermined range. The reason why these factorscan be used in the determination technique or method according to thesecond determination section can be clear from the above-mentionedexplanation of the determination technique according to thedetermination section which determines whether the light oil has spreadin the predetermined range. Thus, a detailed explanation thereof isomitted.

<SOx Poisoning Recovery and Oxidation Removal of PM>

In general, the NOx catalyst has a property that absorbs not only theNOx but also the SOx contained in the exhaust gas. As the amount of theSOx held in the NOx catalyst increases, so-called SOx poisoning iscaused in which the capability of absorbing NOx is decreased.Accordingly, in order to eliminate such SOx poisoning, the processing ofremoving the SOx held in the NOx catalyst through the release andreduction thereof (SOx poisoning recovery processing) is carried out atappropriate times. Additionally, in general, in case where the NOxcatalyst also has a filter function, as in the case when the NOxcatalyst is in the form of a particulate filter carrying theabove-mentioned NOx storage reduction catalyst for example, theprocessing of oxidatively removing captured particulate materials (PM:particulate matter) (PM oxidation removal processing) is timely carriedout.

When these SOx poisoning recovery processing and PM oxidation removalprocessing are performed, it is necessary to raise the temperature ofthe NOx catalyst to a high temperature (e.g., 600 degrees). Thus, toperform SOx poisoning recovery and PM oxidation removal over the entirearea of the NOx catalyst, it is necessary to make the entire area of theNOx catalyst to the high temperature.

Accordingly, in this embodiment, when these processing operations arecarried out, the path through which the exhaust gas flows is alternatelyswitched between the first exhaust path 31 and the second exhaust path32 by means of the switch valve 35. Here, note that such switching needonly to be done at least one time. As a result, in each of the exhaustpaths, the exhaust gas changes, at least one time, from a state in whichthe SV is small to a state in which the SV is high (or vice versa).Accordingly, by injecting the light oil from the addition valve 37during such time, it is possible to supply the light oil to all aroundthe entire areas of the NOx catalysts 33, 34. As a consequence, theentire areas of the NOx catalysts 33, 34 can be uniformly made at a hightemperature.

Here, reference will be made to the driving timing of the switch valve35 and the injection timing of the light oil by the addition valve 37when these processing operations are carried out while referring to FIG.4. FIG. 4 is a timing chart that illustrates the relation between avalve driving pulse sent to the switch valve 35 and an addition pulsesent to the addition valve 37. When the addition pulse is turned on orat a high level, the light oil is injected by the addition valve 37,whereas when the addition pulse is turned off or at a low level, theaddition valve 37 is stopped so the light oil is not injected. Also,when the valve drive pulse is at 1 (high), only the entrance of the flowpassage for the first exhaust path 31 is opened by the switch valve 35,whereas when the valve drive pulse is at 2 (low), only the entrance ofthe flow passage for the second exhaust path 32 is opened by the switchvalve 35.

FIG. 4A represents a preferred or appropriate example. According to thetiming chart illustrated in FIG. 4A, the light oil is injected by theaddition valve 37 in synchronization with the timing at which the paththrough which the exhaust gas flows is switched to the first exhaustpath 31, and to the second exhaust path 32. In this case, substantiallythe same amounts of light oil can be supplied to the first exhaust path31 and the second exhaust path 32, respectively, under the condition ofthe same flow rate of the exhaust gas. Accordingly, appropriateprocessing can be done with respect to both of the NOx catalysts 33, 34.

On the other hand, FIG. 4B represents an inappropriate example.According to the timing chart illustrated in FIG. 4B, the light oil isinjected by the addition valve 37 in synchronization with the timing atwhich the path through which the exhaust gas flows is switched to thefirst exhaust path 31 alone. In this case, the amounts of light oil tobe supplied to the first exhaust path 31 and the second exhaust path 32are different from each other, and the flow rates of the exhaust gaswhen the light oil is supplied to the first and second flow paths arealso different from each other. Accordingly, appropriate processing cannot be done with respect to the NOx catalysts 33, 34.

<Advantageous Effects Achieved by the Internal Combustion EngineProvided with the Exhaust Gas Purification Device According to thisEmbodiment>

As described in the foregoing, according to the internal combustionengine provided with the exhaust gas purification device and the exhaustemission control method for an internal combustion engine according tothis embodiment, when the processing of releasing and reducing the NOxheld in the NOx catalysts 33, 34 is carried out, the droplet-like lightoil can be easily adhered to all around the entire areas of the NOxcatalysts 33, 34, so the region of the reducing atmosphere formed byindividual droplets of the light oil can be widened, and the state ofthe reducing atmosphere can be maintained for a long period of time. Inaddition, the temperature of the NOx catalyst goes up quickly or at anearly stage, so that the releasing and reducing speed or rate of NOx dueto the NOx catalyst can be enhanced. Accordingly, the NOx held in theNOx catalysts 33, 34 can be efficiently reduced and purified, and asufficient amount of NOx can be reduced.

In addition, the NOx catalysts 33, 34 can be regenerated over their wideranges or areas to a satisfactory extent.

<Others>

In this embodiment, as a processing method of decreasing the flow rateof the exhaust gas, there is adopted the method of arranging two exhaustpaths and adjusting the flow rate of the exhaust gas to each of theexhaust paths. However, it is needless to say that three or more exhaustpaths can be provided to decrease the flow rate of the exhaust gas byadjusting the flow rate of the exhaust gas to each of the exhaust paths.In addition, as a processing method or scheme of decreasing the flowrate of the exhaust gas, other than the above, there are enumerated aconstruction that adopts a variable valve system, a construction inwhich the amount of intake air and/or the amount of exhaust gas can beadjusted by intake and/or exhaust valves, a construction in which theamount of EGR is adjusted by an EGR valve, and a construction in whichthe amount of intake air is adjusted by a throttle valve. Specifically,for example, the flow rate of the exhaust gas can be decreased byshortening the valve-opening duration of each of intake and exhaustvalves by means of a variable valve system, or by adjusting a throttlevalve to its closing side and an EGR valve to its opening side, or bysqueezing an exhaust throttle valve (=a valve arranged in an exhaustpassage: this is different from a so-called VVT exhaust valve which isinstalled on a truck, etc., so that it is throttled so as to be used asan engine brake at the time of deceleration).

In addition, in this embodiment, after the injection of the light oil bythe addition valve 37 is terminated, the processing of decreasing theflow rate of the exhaust gas is performed. This is mainly due to theviewpoint of eliminating unnecessary consumption of the light oil, butthe injection of the light oil by the addition valve 37 may be somewhatcontinued after the processing of decreasing the flow rate of theexhaust gas has been started.

Moreover, in this embodiment, there is shown the construction in whichthe switch valve 35 for switching the path through which the exhaust gasflows between the first exhaust path 31 and the second exhaust path 32is arranged at a branch point upstream of these exhaust paths, but sucha switch valve for switching the flow path of the exhaust gas betweenthe exhaust paths may be arranged at a confluence or junction pointdownstream of these exhaust paths. The former construction is better inguiding the light oil to a desired one of the exhaust paths in areliable manner, but the latter construction is better when consideringthe environmental temperature.

Further, in this embodiment, by arranging the addition valve 37 in theexhaust manifold, the distances from the addition valve 37 to the NOxcatalysts 33, 34 can be made long enough. As a result, the temperatureof the fuel in the form of the light oil injected from the additionvalve 37 rises to a satisfactory extent, so the light oil becomes areadily vaporable or evaporable state. In addition, the addition valve37 is arranged at a location upstream of a turbo 38. Accordingly, thefuel flowing into the turbo 38 is stirred therein, so the fuel can bemade to reach the NOx catalysts 33, 34 in a relatively uniform manner.

1. An exhaust gas purification device for an internal combustion enginein which a reducing agent supply means is disposed on an exhaust passagefor supplying a droplet-like reducing agent to a NOx storage reductioncatalyst, which serves to occlude and reduce NOx components in anexhaust gas, from its upstream side, so that NOx components held in saidNOx catalyst are reduced and purified by the reducing agent suppliedthereto by said reducing agent supply means, said device beingcharacterized by comprising: a determination means for determiningwhether the droplet-like reducing agent supplied by said reducing agentsupply means has spread to at least a predetermined range; and anadjustment means for adjusting a flow rate of the exhaust gas sent tosaid NOx catalyst; wherein the flow rate of the exhaust gas is decreasedby said adjustment means when said determination means makes adetermination that said reducing agent has spread.
 2. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 1, characterized in that when said determination means makes adetermination that said reducing agent has spread, the supply of thereducing agent by said reducing agent supply means is stopped, andthereafter the flow rate of the exhaust gas is decreased by saidadjustment means.
 3. The exhaust gas purification device for an internalcombustion engine as set forth in claim 1, characterized in that anelement which becomes a criterion for the determination of saiddetermination means is a NOx purification rate of said NOx catalyst. 4.The exhaust gas purification device for an internal combustion engine asset forth in claim 1, characterized in that an element which becomes acriterion for the determination of said determination means is HCexhausted to a downstream side of said NOx catalyst.
 5. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 1, characterized in that an element which becomes a criterion forthe determination of said determination means is a temperature of saidNOx catalyst.
 6. The exhaust gas purification device tor an internalcombustion engine as set forth in claim 1, characterized in that anelement which becomes a criterion for the determination of saiddetermination means is a time elapsed from the start of supply of thereducing agent by said reducing agent supply means.
 7. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 6, characterized in that said determination means determines thatthe reducing agent has spread in the predetermined range when the timeelapsed from the start of supply of the reducing agent by said reducingagent supply means exceeds a predetermined time.
 8. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 7, characterized in that said predetermined time is a presetreference time, or a time that is set based on said reference time inconsideration of a flow rate of the exhaust gas having passed through aunit volume of said catalyst per unit time.
 9. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 1, characterized in that an element which becomes a criterion forthe determination of said determination means includes at least one of aNOx purification rate of said NOx catalyst, HC exhausted to a downstreamside of said NOx catalyst, a temperature of said NOx catalyst, a timeelapsed from the start of supply of the reducing agent by said reducingagent supply means, and a flow rate of the exhaust gas having passedthrough a unit volume of said catalyst per unit time.
 10. The exhaustgas purification device for an internal combustion engine as set forthin claim 1, characterized by further comprising a second determinationmeans for determining whether the adjustment of decreasing the flow rateof the exhaust gas by said adjustment means is to be terminated.
 11. Theexhaust gas purification device for an internal combustion engine as setforth in claim 10, characterized in that an element which becomes acriterion for the determination of said second determination means isthe NOx purification rate of said NOx catalyst.
 12. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 10, characterized in that an element which becomes a criterion forthe determination of said second determination means is HG exhausted tothe downstream side of said NOx catalyst.
 13. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 10, characterized in that an element which becomes a criterion forthe determination of said second determination means is the temperatureof said NOx catalyst.
 14. The exhaust gas purification device for aninternal combustion engine as set forth in claim 10, characterized inthat an element which becomes a criterion for the determination of saidsecond determination means is a time elapsed from the start of theadjustment of decreasing the flow rate of the exhaust gas by saidadjustment means.
 15. The exhaust gas purification device for aninternal combustion engine as set forth in claim 14, characterized inthat said second determination means determines that the adjustment ofdecreasing the flow rate of the exhaust gas by said adjustment means isto be terminated when the time elapsed from the start of the adjustmentof decreasing the flow rate of the exhaust gas by said adjustment meansexceeds a second predetermined time.
 16. The exhaust gas purificationdevice for an internal combustion engine as set forth in claim 15,characterized in that said second predetermined time is a presetreference time, or a time that is set based on said reference time inconsideration of a flow rate of the exhaust gas having passed through aunit volume of said catalyst per unit time.
 17. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 10, characterized in that an element which becomes a criterion forthe determination of said second determination means includes at leastone of a NOx purification rate of said NOx catalyst, HC exhausted to adownstream side of said NOx catalyst, a temperature of said NOxcatalyst, a time elapsed from the start of the adjustment of decreasingthe flow rate of the exhaust gas by said adjustment means, and a flowrate of the exhaust gas having passed through a unit volume of saidcatalyst per unit time.
 18. The exhaust gas purification device for aninternal combustion engine as set forth in claim 1, characterized inthat the lower the temperature of said NOx catalyst, the more the flowrate of the exhaust gas is decreased by said adjustment means.
 19. Theexhaust gas purification device for an internal combustion engine as setforth in claim 1, characterized by further comprising: a first exhaustpath and a second exhaust path arranged at a downstream side of saidreducing agent supply means, with a NOx catalyst being provided on eachof said first and second exhaust paths; and a valve that adjusts theflow rate of the exhaust gas with respect to each of said exhaust paths;wherein when the processing of reducing and purifying the NOx held inthe NOx catalysts is not performed, the exhaust gas is caused to flowinto each of said exhaust paths; when said purification processing isperformed, the supply of the reducing agent to one of said NOx catalystsby means of said reducing agent supply means is started with the exhaustgas being controlled by said valve to flow only into that one of saidexhaust paths in which the one of said NOx catalysts to be processed forpurification is arranged, and when the processing of decreasing the flowrate of the exhaust gas by means of said adjustment means is performed,the exhaust gas is controlled to flow into the other of said exhaustpaths by said valve, whereby the flow rate of the exhaust gas to the oneof said exhaust paths in which the one of said NOx catalysts to beprocessed for purification is arranged is decreased.
 20. The exhaust gaspurification device for an internal combustion engine as set forth inclaim 19, characterized in that when SOx held in said NOx catalysts isreduced and purified, and when particles adhered to said NOx catalysts,which also have a filter function, are oxidatively removed, processingof increasing and decreasing the flow rate of the exhaust gas flowingthrough that one of said exhaust paths in which one of said NOxcatalysts to be processed for purification is arranged is performed bysaid valve at least one time.
 21. The exhaust gas purification devicefor an internal combustion engine as set forth in claim 20,characterized in that said valve comprises a switch valve that is ableto switch a path through which the exhaust gas flows to said firstexhaust path or said second exhaust path; said increasing and decreasingprocessing is performed by said switch valve that alternately switchesthe path through which the exhaust gas flows between said first andsecond exhaust paths; and the timing at which the reducing agent issupplied by said reducing agent supply means is synchronized with thetiming at which the path through which the exhaust gas flows is switchedby said switch valve.
 22. The exhaust gas purification device for aninternal combustion engine as set forth in claim 21, characterized inthat after the reducing agent is started to be supplied by said reducingagent supply means in synchronization with the timing at which the paththrough which the exhaust gas flows is switched to either one of saidfirst and second exhaust paths by means of said switch valve, the supplyof the reducing agent is stopped during the time when the exhaust gas isflowing through said one of said first and second exhaust paths, andthereafter the reducing agent is started to be supplied by said reducingagent supply means in synchronization with the timing at which the paththrough which the exhaust gas flows is switched to the other of saidfirst and second exhaust paths by said switch valve.
 23. An exhaust gaspurification method for an internal combustion engine for purifying NOxcontained in an exhaust gas, said method comprising: a step of making adroplet-like reducing agent adhere to an occlusion reduction type NOxcatalyst by supplying a reducing agent from an upstream side of said NOxcatalyst that occludes and reduces NOx; and a step of decreasing a flowrate of the exhaust gas sent to said NOx catalyst after it is determinedby a determination means that said droplet-like reducing agent hasspread in at least a predetermined range in said NOx catalyst.